This invention relates generally to a drive mechanism for a positioning system.
Harmonic drive gearing is a single-stage gear transmission having three main components. These components are known as the circular spline, the flexspline, and the wave generator. The circular spline is a circular, rigid sun gear with teeth on its inner surface. The flexspline is a thin-walled cup with teeth on its outer surface, which are designed to mesh with the teeth of the circular spline. The flexspline is radially flexible and torsionally rigid, and has fewer teeth and a smaller radius than the circular spline. The wave generator is a non-circular cam, which fits inside the flexspline and deforms it to cause some of its teeth to mesh with the teeth of the circular spline and others to completely unmesh. Each full rotation of the wave generator causes the flexspline to walk around the circular spline at a rate equal to the difference in the number of teeth of the flexspline to the number of teeth of the circular spline. For example, if the circular spline has 100 teeth and the flexspline has 98 teeth, each revolution of the wave generator will walk the flexspline around the circular spline a distance equal to two teeth.
Industries benefiting from harmonic drive gearing include semiconductor, machine tool, factory automation, robotics, medical equipment and aerospace. Medical applications benefiting from harmonic drive gearing include patient beds, rehabilitations equipment, and MRI/CAT scan gantries. Other uses for harmonic drives include radiation therapy equipment, imaging camera positioning and surgical robots. Robots used in semiconductor component manufacturing use harmonic drives to accurately position wafers for processing, loading, unloading, inspection and test. Military and aerospace applications of harmonic drive gearing include communication, military surveillance, and weather satellites, several deep space probes, telescopes including the Hubble Space Telescope, and the International Space Station. Harmonic Drives are used to accurately control antennas and compass gimbals, to align scientific instruments, adjust apertures and solar panels and open and close hatches and doorways.
These applications require high positional accuracy, repeatability, and low vibration. Harmonic drive gear sets are ideal for precision applications that require a compact design and high torque to weight ratio. They are capable of less than one arc minute positional accuracy and repeatability of +/−5 arc seconds without the benefit of feedback at the output stage.
The traditional design for harmonic drive gearing involves a flexspline with two fewer teeth than the circular spline and an elliptical wave generator designed such that it causes the teeth of the flexspline to mesh with the teeth of the circular spline in two zones, diametrically opposed to each other and corresponding to the major axis of the ellipse. The minor axis of the wave generator is short enough that it allows the teeth of the flexspline to completely unmesh in the zones along the minor axis and allows a one-tooth difference in length between the circular spline and the flexspline in the areas of no contact. The torque capacity of the gearing system is equal to the shear force necessary to cause the teeth in contact to fail. This so allows the existence of a “ratcheting torque,” under which the flexspline may slip by a distance of one tooth relative to the circular spline. This increases the number of teeth in contact and therefore the torque capacity and the torsional rigidity. However, this condition greatly increases the forces on the flexspline, greatly reducing its fatigue life. Furthermore, it throws the system out of balance, greatly increasing noise and vibration and reducing positional accuracy.